Historically, today's most favorite rechargeable energy storage devices—lithium-ion batteries—was actually evolved from rechargeable “lithium metal batteries” that use lithium (Li) metal as the anode and a Li intercalation compound (e.g. MoS2) as the cathode. Li metal is an ideal anode material due to its light weight (the lightest metal), high electronegativity (−3.04 V vs. the standard hydrogen electrode), and high theoretical capacity (3,860 mAh/g). Based on these outstanding properties, lithium metal batteries were proposed 40 years ago as an ideal system for high energy-density applications.
Due to some safety concerns of pure lithium metal, graphite was implemented as an anode active material in place of the lithium metal to produce the current lithium-ion batteries. The past two decades have witnessed a continuous improvement in Li-ion batteries in terms of energy density, rate capability, and safety. However, the use of graphite-based anodes in Li-ion batteries has several significant drawbacks: low specific capacity (theoretical capacity of 372 mAh/g as opposed to 3,860 mAh/g for Li metal), long Li intercalation time (e.g. low solid-state diffusion coefficients of Li in and out of graphite and inorganic oxide particles) requiring long recharge times (e.g. 7 hours for electric vehicle batteries), inability to deliver high pulse power, and necessity to use pre-lithiated cathodes (e.g. lithium cobalt oxide, as opposed to cobalt oxide), thereby limiting the choice of available cathode materials. Further, these commonly used cathode active materials have a relatively low lithium diffusion coefficient (typically D˜10−16−10−11 cm2/sec). These factors have contributed to one major shortcoming of today's Li-ion batteries—a moderate energy density (typically 150-220 Wh/kgcell), but extremely low power density (typically <0.5 kW/kg).
Supercapacitors are being considered for electric vehicle (EV), renewable energy storage, and modern grid applications. The relatively high volumetric capacitance density of a supercapacitor (10 to 100 times greater than those of electrolytic capacitors) derives from using porous electrodes to create a large surface area conducive to the formation of diffuse double layer charges. This electric double layer capacitance (EDLC) is created naturally at the solid-electrolyte interface when voltage is imposed. This implies that the specific capacitance of a supercapacitor is directly proportional to the specific surface area of the electrode material, e.g. activated carbon. This surface area must be accessible by the electrolyte and the resulting interfacial zones must be sufficiently large to accommodate the EDLC charges.
This EDLC mechanism is based on surface ion adsorption. The required ions are pre-existing in a liquid electrolyte and do not come from the opposite electrode. In other words, the required ions to be deposited on the surface of a negative electrode (anode) active material (e.g., activated carbon particles) do not come from the positive electrode (cathode) side, and the required ions to be deposited on the surface of a cathode active material do not come from the anode side. When a supercapacitor is re-charged, local positive ions are deposited close to a surface of a negative electrode with their matting negative ions staying close side by side (typically via local molecular or ionic polarization of charges). At the other electrode, negative ions are deposited close to a surface of this positive electrode with the matting positive ions staying close side by side. Again, there is no exchange of ions between an anode active material and a cathode active material.
In some supercapacitors, the stored energy is further augmented by pseudo-capacitance effects due to some local electrochemical reactions (e.g., redox). In such a pseudo-capacitor, the ions involved in a redox pair also pre-exist in the same electrode. Again, there is no exchange of ions between the anode and the cathode.
Since the formation of EDLC does not involve a chemical reaction or an exchange of ions between the two opposite electrodes, the charge or discharge process of an EDL supercapacitor can be very fast, typically in seconds, resulting in a very high power density (typically 3-10 kW/Kg). Compared with batteries, supercapacitors offer a higher power density, require no maintenance, offer a much higher cycle-life, require a very simple charging circuit, and are generally much safer. Physical, rather than chemical, energy storage is the key reason for their safe operation and extraordinarily high cycle-life.
Despite the positive attributes of supercapacitors, there are several technological barriers to widespread implementation of supercapacitors for various industrial applications. For instance, supercapacitors possess very low energy densities when compared to batteries (e.g., 5-8 Wh/kg for commercial supercapacitors vs. 10-30 Wh/Kg for the lead acid battery and 50-100 Wh/kg for the NiMH battery). Modern lithium-ion batteries possess a much higher energy density, typically in the range of 150-220 Wh/kg, based on the cell weight.
In addition to lithium-ion cells, there are several other different types of batteries that are widely used in society: alkaline Zn/MnO2, nickel metal hydride (Ni-MH), lead-acid (Pb acid), and nickel—cadmium (Ni—Cd) batteries. Since their invention in 1860, alkaline Zn/MnO2 batteries have become a highly popular primary (non-rechargeable) battery. It is now known that the Zn/MnO2 pair can constitute a rechargeable battery if an acidic salt electrolyte, instead of basic (alkaline) salt electrolyte, is utilized. However, the cycle life of alkaline manganese dioxide rechargeable batteries has been limited to typically 20-30 cycles due to irreversibility associated with MnO2 upon deep discharge and formation of electrochemically inactive phases.
Additionally, formation of a haeterolite (ZnO:Mn2O3) phase during discharge, when Zn penetrates into the lattice structure of MnO2, has made battery cycling irreversible. The Zn anode also has limitations on cycle life due to the redistribution of Zn active material and formation of dendrites during recharge, causing internal short-circuits. Attempts to solve some of these issues have been made by Oh, et al. [S. M. Oh, and S. H. Kim, “Aqueous Zinc Sulfate (II) Rechargeable Cell Containing Manganese (II) Salt and Carbon Powder,” U.S. Pat. No. 6,187,475, Feb. 13, 2001] and by Kang, et al. [F. Kang, et al. “Rechargeable Zinc Ion Battery”, U.S. Pat. No. 8,663,844, Mar. 4, 2014]. However, long-term cycling stability and power density issues remain to be resolved. Due to these reasons, the commercialization of this battery has been limited.
Xu, et al. US Pub. No. 20160372795 (Dec. 22, 2016) and US Pub. No. 20150255792 (Sep. 10, 2015) reported Ni-ion and Zn-ion cells, respectively, which both make use of graphene sheets or carbon nanotubes (CNTs) as the cathode active material. Although these two patent applications claim an abnormally high specific capacity of 789-2500 mAh/g based on the cathode active material weight, there are several serious problems associated with these two cells:    (1) There is no plateau portion in the charge or discharge curves (voltage vs. time or voltage vs. specific capacity), unlike typical lithium-ion batteries. This lack of a voltage curve plateau means the output voltage being non-constant (varying too much) and would require a complicated voltage regulation algorithm to maintain the cell output voltage at a constant level.    (2) Actually, the discharge curve for the Ni-ion cell exhibits an extremely sharp drop in voltage from 1.5 volts to below 0.6 volts as soon as the discharge process begins and, during most of the discharge process, the cell output is below 0.6 volts, which is not very useful. As a point of reference, the alkaline cell (a primary battery) provides an output voltage of 1.5 volts.    (3) The discharge curves are characteristic of surface adsorption or electroplating mechanisms at the cathode, as opposed to ion intercalation. Further, it appears that the main event that occurs at the cathode during the battery discharge is electroplating. The high specific capacity values reported by Xu, et al. are simply a reflection on the high amount of Ni or Zn metal electroplated on the surfaces of graphene or CNTs. Since there is an excess amount of Ni or Zn in the anode, the amount of electroplated metal increases as the discharge time increases. Unfortunately, the electrochemical potential difference between the anode and the cathode continues to decrease since the difference in the metal amount between the anode and the cathode continues to decrease (more Zn or Ni is dissolved from the anode and gets electroplated on cathode surfaces). This is likely why the cell output voltage continues to decrease. The cell voltage output would be essentially zero when the amounts of metal at the two electrodes are substantially equivalent or identical. Another implication of this electroplating mechanism is the notion that the total amount of the metal that can be deposited on the massive surfaces at the cathode is dictated by the amount of the metal implemented at the anode when the cell is made. The high specific capacity (as high as 2,500 mAh/g) of graphene sheets at the cathode simply reflects the excessively high amount of Zn provided in the anode. There is no other reason or mechanism for why graphene or CNTs could “store” so much metal. The abnormally high specific capacity values as reported by Xu, et al. were artificially obtained based on the high amounts of Ni or Zn electroplated on cathode material surfaces, which unfortunately occurred at very low voltage values and are of little utility value.
Clearly, an urgent need exists for new cathode materials that provide proper discharge voltage profiles (having a high average voltage and/or a high plateau voltage during discharge), high specific capacity at both high and low charge/discharge rates (not just at a low rate), and long cycle-life for a multivalent metal secondary battery. Hopefully, the resulting battery can deliver some positive attributes of a supercapacitor (e.g. long cycle life and high power density) and some positive features of a lithium-ion battery (e.g. moderate energy density). These are the main objectives of the instant invention.